Vehicle mounted aerial devices are commonly used to perform work on utility poles, utility lines and other overhead equipment. Typically, the aerial device has a boom assembly which is mounted for rotation on the bed of a utility vehicle. A personnel carrying platform is mounted on the end of the boom assembly to allow workers to be properly positioned for the job that faces them. The boom assembly can be either a telescoping assembly or an articulating assembly in which upper and lower booms are pivoted together end to end. The present invention relates to articulating boom assemblies.
Articulating booms may be equipped with what is commonly known as an upper boom compensation system. The compensation system automatically actuates the upper boom cylinder or mechanically moves it when the lower boom is moved, thus assuring that the desired positioning of the upper boom and platform accompanies lower boom movement. An upper boom compensation aerial device effectively reduces the time required to place the operator at the desired work location, thus making the operator more productive, and reduces the amount of control valve manipulation by the operator, thereby simplifying his job. Different types of systems have been proposed to achieve upper boom compensation.
In one type of compensation system, a sliding link mechanism is mounted in the lower boom and attached at a location offset from the lower boom pivot point. The base of the upper boom cylinder is connected with the slide mechanism such that the upper boom cylinder base moves when the lower boom is pivoted up and down. This movement of the upper boom cylinder base causes the upper boom cylinder, acting as a fixed link, to pivot the upper boom up and down to correspond with up and down movement of the lower boom. Actuation of the upper boom cylinder also allows operation of the upper boom independently of the lower boom.
Another known type of boom compensation system makes use of a parallelogram arrangement which includes the lower boom and a rigid link that is maintained parallel to the lower boom. The parallelogram configuration causes the upper boom to maintain its attitude relative to horizontal as the lower boom is raised and lowered. An upper boom cylinder allows the upper boom to be raised and lowered independently of the lower boom.
These mechanical systems have functioned in a satisfactory manner for the most part. However, they involve complicated mechanics and the usual problems associated with mechanical equipment that can be subjected to significant loads. The parts are susceptible to wear which can inhibit their performance, particularly after prolonged service. The repair and maintenance requirements can be considerable. The need to provide a complicated mechanical system can be a significant factor in the overall cost of the machine.